During the manufacture of multi-layer printed wiring boards (PWBs), the assimilation of several patterned conductive copper layers requires separation by dielectric materials. To build interconnecting pathways between the copper layers, through-holes are drilled into which conductive material, copper, is plated to cover the dielectric material and connect the innerlayers of copper. Current methods for plating copper throughout the drilled holes involve making the through-holes conductive to enable subsequent electrolytic copper plating. The most common method of generating a conductive copper layer in the through-holes is electroless copper plating whereby formaldehyde is used as an electron source to reduce copper ions to copper metal within the through-holes. To direct the copper reduction to the walls of the through-holes, several processing steps ensure coating of the through-holes with catalyst material. Current catalyst solution baths, into which drilled multi-layer boards are dipped during processing, contain palladium as the active ingredient. The cost of palladium has increased dramatically over the years. Thus substitution of the palladium containing catalyst solution with an inexpensive alternative has been a goal of the electroless plating industry for some time.
While many non-palladium catalyst solutions are capable of initiating electroless copper plating, replicating the performance behavior of current commercial palladium systems has proven challenging. The most critical factors influencing the attractiveness of alternative catalyst solutions include: Stability—Catalyst solution baths must retain their activity over the course of several months in open air at process temperatures. Activity—Through-holes removed from the electroless plating baths, following exposure to the catalyst bath, must contain full and uniform copper coverage and be sufficiently conductive to enable electrolytic plating. If alternative catalyst solutions require substantial increases in cycle times, raw material cost benefits are likely to be offset by dramatic through-put reductions. Thus, alternative catalyst solutions must provide sufficient activity within the constraints of current process flows. Interconnect quality—Processed, post-electrolytic copper plating, multi-layer boards must possess sufficient electrical conductivity throughout the multi-channel copper containing pathways within the board. While a host of factors, such as poor drilling or insufficient through-hole preparation, may contribute to defects occurring at the interface of the copper layers and the conducting through-holes, defects inherent to the use of a particular catalyst are unacceptable.
The use of silver serves to address the raw material price and volatility issues facing current palladium based systems. While catalyst solutions based on silver have previously been reported, none have had commercial success due to failures in any one of the factors described above. Most typically, silver catalyst solutions fail to provide sufficient activity within the through-holes for electroless copper plating. This may either be a function of the amount of catalyst absorbed within the through-holes or the rate of initiation of electroless plating provided by the catalyst; in either case, the result is insufficient copper coverage within the through-holes for subsequent electrolytic plating. It is likely that many factors influence the activity provided by a particular silver catalyst solution, namely silver particle size, choice of stabilizing agent, and the presence of components such as pH buffers and other additives.
Patent publication U.S. 2004/0043153 to Okuhama et al. discloses a silver colloid solution produced by reducing a silver ion by an ion of a metal having an electric potential which can reduce the silver ion to silver metal. The solution also includes one or more ions selected from a hydroxycarboxylate ion, a condensed phosphate ion and an amine carboxylate ion. The silver colloid may also include one or more ions selected from the metals of atomic numbers 26 to 30. The patent publication discloses that the silver colloid may be used as a catalyst for electroless plating. Although there are silver colloids which may be used as catalysts for electroless plating, there is still a need for an improved silver catalyst to electrolessly plate metal on non-conductive substrates.